He is generally credited for doing the first serious scientific research on the Atkins Diet. He has published articles on application of thermodynamics to nutrition. His articles explain why the common idea that "a calorie is a calorie" is not correct.

His argument in brief: the first law of thermodynamics (conservation of energy) is a bookkeeping law, that it does not say how energy input is divided between weight gain, work done, heat generation or storage of energy in different biomolecules. The second law is a dissipation law. It says that all (real) processes are inefficient.

Feinman explained how diets of different composition vary in efficiency, in particular, how carbohydrate-restricted diets can lead to reduced efficiency. Whereas the effect is not always seen, the argument is that there is no reason to doubt published reports where it does occur.

He is a director of the Metabolism Society and co-Editor-in-Chief of the Open Access online medical journal, Nutrition & Metabolism.